The main thing that bothers me about this is the change and ambitious nature of the wording.
In the days of tennis balls and cane tips- the rule was if you shouldered your pistol, then it was an sbr-- so this would be very easy to figure out, shoulder it- bad, not shoulder it- ok. Pretty cut and dry
But with this new "rule interpretation" you no longer even have to shoot your pistol for it to be considered an sbr
It is no longer cut and dry, its up to interpretation. If the caliber/cartridge is too lage then its not a pistol, if the optics are not designed for pistols then it's not a pistol, if the "lop" is too long its not a pistol, if its too heavy its not a pistol.
Yet you see they don't actually give you any numbers for lop, weight, cartridge-- its all subjective .
After they get away with this, I expect they will carry it further-- if it has a handguard or forend then it won't be a pistol, then maybe "all bottleneck cartridges" won't be allowed as pistols, then what-- its a slippery slope
One of your options is to et the "free tax stamp" and register it, I'm guessing that any ar style chassis firearm will be "re-classified" as nfa items and need to be registered in the future
But you see that what happens with pistol braces could just be generated to pistol classification, which could even roll over to breach or bolt action pistols that fire "rifle cartridges" in the future.